1. Field of the Invention
The present invention deals with baking lacquers for single and double layer lacquering on the basis of a blend of hydroxy-group-containing polymers, blocked polyisocyanates and organic solvents.
2. Description of the Prior Art
For the production of mixtures of polyisocyanates and polyhydroxy-compounds that are storable at normal temperatures, it is known to use such polyisocyanates which reactive groups are blocked by reaction with nonfunctional groups and that are generally described as blocked, capped or masked polyisocyanates. Such products are described in Liebig's Annalen Vol. 562, pp 205-229. Through the influence of higher temperatures, they dissociate back into their original materials, whereby the, now freed, isocyanate groups can, on their part, react with the polyhydroxy-compounds that are present in the reaction mixture.
Such a single-component baking lacquer system that is storage-stable at room temperature, for example, consists of an .epsilon.-caprolactam-blocked adduct of 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexyl isocyanate, dissolved in a high-boiling lacquer solvent, and a hydroxyl-group containing polyester on the basis of predominantly aromatic dicarboxylic acids and a mixture of diols and triols and is described in the DT-OS (German Patent Disclosure) No. 23 46 818. But for certain applications, the hardening speeds of such a system are too slow. Although it is possible to considerably increase the hardening speeds with such catalysts as tertiary amines or organic tin compounds, such additives influence, especially in the presence of compounds that are capable of reacting with isocyanates, the capped polyisocyanates--even at low temperatures, in such a way, that they cause the splitting-off of the monofunctional compound, even at normal temperature and thus render the mixture unstable.